1. Field of the Invention
The invention generally relates to a lens, and more particularly, to a fixed-focus lens.
2. Description of Related Art
Along with the development of display technology, a new generation of displays, such as liquid crystal display (LCD), plasma display panel (PDP) and projection apparatus, gradually substitute traditional cathode ray tub (CRT), wherein the flat displays such as LCD and PDP occupy a larger portion in the household appliances market due to the thinner figure thereof. On the other hand, the projection apparatus keeps occupy a considerable portion in the market due to the capability of providing a mega-size frame (larger than 52 inch, for example) with a lower cost. The above-mentioned mega-size frame is suitable for many viewers to watch, and it is valuable for holding conference, report presentation or teaching information demo, which becomes one of major reasons for the projection apparatus to be unsubstitutional as well. In recent years, the projection apparatus even plays an indispensable role to establish a home theater.
In a projection apparatus, the image on a light valve is a small but dedicate frame, wherein the light valve is, for example, liquid-crystal-on-silicon panel (LCOS panel) or digital micro-mirror device (DMD). In order to project the small frame of the light valve on a screen to obtain a large frame, a projection lens is required. The projection lens makes the image beam from the light valve projected on the screen, wherein to make the projection apparatus able to produce a larger projected image frame in a shorter distance, a fixed-focus lens with a larger field of view (FOV), i.e., a wide-angle lens, is needed.
During designing a wide-angle lens, a designer must puzzle the aberration issue. In order to reduce aberration of the wide-angle lens, an aspheric lens may be employed to correct the aberration as the regular solution. However, the cost and the production difficulty of the aspheric lens are higher. The aspheric lens also makes the assembly of the lens more difficult. The more the employed aspheric lenses, the higher the fabrication difficulty and the cost are. If a designer wishes to employ a less number of the aspheric lenses and keep the aberration within an acceptable range, the length of the conventional wide-angle lens may be increased or more lenses are required to correct the aberration in the prior art. A larger total length of the wide-angle lens results in a larger volume of the projection apparatus and an expensive cost.